The configuration of a conventional ultrasonic transducer is shown in FIG. 18. Most conventional ultrasonic transducers are resonant ultrasonic transducers using a piezoelectric ceramic as a vibrating element. The ultrasonic transducer shown in FIG. 18 uses the piezoelectric ceramic as the vibrating element to perform both conversion from an electric signal to ultrasonic waves and conversion from ultrasonic waves to the electric signal (transmission and reception of ultrasonic waves). The bimorph-type ultrasonic transducer shown in FIG. 18 comprises two piezoelectric ceramics 61 and 62, a cone 63, a case 64, leads 65 and 66, and a screen 67.
The piezoelectric ceramics 61 and 62 are stuck together, and the leads 65 and 66 are respectively connected to the ceramics 61 and 62 at the surfaces thereof opposite to the stuck surface.
Since the resonant ultrasonic transducer uses a resonance phenomena of the piezoelectric ceramic, excellent ultrasonic transmission and reception characteristics can be obtained only in a relatively narrow frequency band near the resonance frequency.
In addition to the resonant ultrasonic transducer shown in FIG. 18, the electrostatic ultrasonic transducer has been heretofore known as a broadband oscillation-type ultrasonic transducer which can generate relatively high sound pressure over a wide frequency band.
However, as shown in FIG. 19, regarding the maximum value of the sound pressure, the electrostatic ultrasonic transducer has a value as low as 120 dB or lower as shown by the curve Q1 in FIG. 19 while the resonant ultrasonic transducer has a value as high as 130 dB or higher as shown by the curve Q2 in FIG. 19. Hence the sound pressure is slightly insufficient for using the electrostatic ultrasonic transducer as an ultrasonic speaker. Such ultrasonic speaker using the electrostatic ultrasonic transducer is disclosed in, for example, Published Japanese translation Nos. 2002-526004 and 2004-501524 of PCT International Applications.
Here, explanation will be given of the ultrasonic speaker in which the ultrasonic transducer is utilized. In the ultrasonic speaker, an ultrasonic wave referred to as a carrier wave, is AM modulated by an audio signal (a signal in an audio-frequency band), and when this is radiated to the air the original audio signal is self-reproduced in the air due to the nonlinearity of the air.
More specifically, since the sound waves are compression waves that propagate through the air as a medium, dense parts and sparse parts of the air appear remarkably in a process of propagation of the modulated ultrasonic waves. Since the speed of sound is fast in the dense parts and is slow in the sparse parts, a distortion occurs in the modulated wave itself. As a result, the waveform is separated into carrier waves (ultrasonic wave) and audio waves (original audio signal), and a human can hear only the audio sound (original audio signal) of 20 kHz or below. This principle is generally referred to as a parametric array effect.
An ultrasonic sound pressure of not lower than 120 dB is necessary in order that the parametric array effect appears sufficiently, but it is difficult to achieve this figure by the electrostatic ultrasonic transducer. Hence, a ceramic piezoelectric element such as PZT or a polymer piezoelectric element such as PVDF has been used as an ultrasonic wave-transmitting member.
However, the piezoelectric element has a sharp resonance point regardless of the material, and is driven at the resonance frequency and put to practical use as an ultrasonic speaker. Therefore, the frequency domain that can ensure a high sound pressure is quite narrow. That is, it can be said that the piezoelectric element has eventually a narrow-band.
Generally, the maximum audio frequency band of a human being is about 20 Hz to 20 kHz, with a band of about 20 kHz. That is, in the ultrasonic speaker, the original audio signal cannot be demodulated with fidelity, unless a high sound pressure is ensured over the frequency band of 20 kHz in the ultrasonic region.
It can be easily understood that it is difficult to reproduce (demodulate) the broadband of 20 kHz with fidelity with the conventional ultrasonic speaker using the piezoelectric element.
Actually, the ultrasonic speaker using the conventional resonant ultrasonic transducer shown in FIG. 6 has the following problems: (1) the band is narrow and reproduced sound quality is low; (2) if the AM modulation factor is too high, the demodulated sound is distorted, and hence the modulation factor can be increased up to about 0.5 at maximum; (3) if the input voltage is increased (if the volume is increased), vibration of the piezoelectric element becomes unstable, and the sound is distorted When the voltage is further increased, the piezoelectric element itself is likely to be broken; and (4) arraying, enlargement, and miniaturization are difficult, and hence the production cost is high.